This invention relates to a fluidized bed reactor and, more particularly, to a method and system for controlling the cyclone collection efficiency and recycle rate in a fluidized bed reactor.
Fluidized bed reactors, such as gasifiers, steam generators, combustors, and the like, are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. The entrained particulate solids are separated externally of the bed and recycled back into the bed. The heat produced by the fluidized bed is utilized in various applications which results in an attractive combination of high heat release, high sulfur absorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed reactor is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turndown afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a "circulating" fluidized bed process. According to this process, the fluidized bed density is well below that of a typical bubbling fluidized bed, the air velocity is greater than that of a bubbling bed and the flue gases passing through the bed entrain a substantial amount of particulate solids and are substantially saturated therewith.
Also, the circulating fluidized bed is characterized by relatively high solids recycling which makes it insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore decreasing the nitrogen oxides formation. Also, the high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle. The resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.
Most of the circulating bed designs currently being utilized control load by regulating the solids recycle rate and some approach this by reducing the solids inventory from the seal pot, i.e. from the sealing system located between the outlet of the external separating devices and the recycle inlet to the fluidized bed. However, this normally has to be accomplished with a metering cooler, such as a water cooled screw, which adds mechanical complexity and costs penalties in addition to requiring downstream handling equipment.